Printed sheets are often varnished, or coated, to provide a smooth surface. In typical printing machines, this is performed by a conventional varnishing unit disposed after the printing units. When two-sided printing occurs wherein one side of the sheets is varnished after having been earlier printed, problems occur due to pressure from a subsequent printing operation on the opposite side destroying the smooth surface produced by the varnishing. Thus, it would appear to be desirable to print the verso side, i.e., perfect the sheet, before transferring the sheet to the varnishing unit for varnishing.
However, the well-known disadvantages of perfecting a sheet before transferring the sheet to a varnishing unit will be appreciated by those skilled in the art of printing. For example, the recently inked side of the sheet must contact some surface in the varnishing unit, generally another cylinder, so that varnish can be evenly applied to the other, previously printed, side. However, unless the ink is first allowed to dry, it will smear and accumulate on the surface it contacts, particularly if the surface is on a cylinder rotating at high speed. After the varnish is applied, the varnish must likewise be dried before stacking the sheets at a delivery unit. Thus, even if ink-smearing was sufficiently reduced by providing enough ink-drying time, this would lengthen the total drying time since first the ink would have to dry before the varnish could be applied and subsequently dried.
Accordingly, U.S. Pat. No. 4,664,949 (corresponding to German Patent No. DE 3,248,232) assigned to the present assignee solves these problems by simultaneously varnishing one already printed side of the sheet while printing the other. This is accomplished by pressing the varnishing cylinder against the impression cylinder, forming a nip between the cylinders such that as sheets pass through, the pressure enables the printing of one side while the other is varnished. As a result, both sides of the sheets dry while being carried to the delivery station, and there is no transferring of the sheet to another cylinder. However, since the ink-carrying rubber blanket or printing plate on the impression cylinder coincides with the varnishing blanket on the varnishing cylinder, the pressure can vary between the impression cylinder and the varnishing cylinder which results in inconsistent printing quality, particularly when instead of varnishing the entire sheet side, the sheet is only partially or intermittently varnished.
Partial varnishing is ordinarily accomplished by providing gaps, or intervals, between a number of separated varnishing blankets (or a single blanket having gaps or intervals therein) secured around the varnishing cylinder. The pressure between the varnishing cylinder and the impression cylinder fluctuates from a normal level to a reduced level as first a varnishing blanket and then an interval between blankets reaches the nip created by the varnishing and impression cylinders. These pressure fluctuations result in the inconsisent transferring of ink to the sheet in the aforementioned device.